Compressed stranded cable is well known in the art. Examples are disclosed in U.S. Pat. Nos. 3,383,704 and 3,444,684. Such cables are preferred over uncompacted cables for several reasons.
Uncompacted cables require the maximum amount of insulation because the cable diameter is not reduced and because superficial valleys between the outer strands are filled with insulation material. In addition, since uncompacted cables are not generally tight-stranded, extrusion of insulation onto the stranded cable usually forces insulation material into the interstices between the individual strands of the cable. In addition, tension on the individual strands of uncompacted cable is usually unequal, which can result in a propensity of the cable to assume a spiral or sine wave configuration.
U.S. Pat. Nos. 3,383,704 and 3,444,684 disclose an advantageous process and compacted cable wherein a plurality of layer strands are wound about at least one core strand and each layer strand is deformed to form a flattened region along the length of the layer strand while leaving the layer strand substantially circular and without deforming the core strand.
Many different types of stranding machines may be used for stranding layer strands over core strands. Examples of tubular type stranders are disclosed in U.S. Pat. Nos. 3,827,225 and 3,902,307. Rigid frame and circular mil type stranders are shown in U.S. Pat. Nos. 3,280,544, 3,934,395, 3,955,348 and 4,253,298. Double-twist stranders are shown in U.S. Pat. Nos. 3,791,131, 3,945,182 and 4,087,956.
While rigid frame and circular mill type stranders have been found satisfactory in producing compressed stranded cable in sizes larger than AWG 4/0 and when more than nineteen wires are used to form the cable, tubular stranders have been preferred in the compressed stranded cable industry for smaller cables. Normal technology is tubular stranders for seven wire and nineteen wire configurations. The tubular stranders, however, are limited to 1,000 rpm when producing seven wire cable and about 700 rpm when producing nineteen wire cable on the larger twelve wire machines. Although tubular stranders are usually marketed for speeds of 1,000 rpm, it is very difficult to exceed 700 rpm while producing the twelve wire layer without breakout problems. Tubular stranding of nineteen wire cable requires one seven wire machine and one twelve wire machine, resulting in a two-pass production cycle for nineteen wire cable.
Double-twist stranders are designed for bunching. Bunching is the random assembly of any number of wires, by simply twisting the single ends together. Stranding is geometrically controlled assembly of the wires in layers, each wire being guided into a specific location within its layer. The capital expense of one buncher is about half that of the two tubular machines which would be required for the same production. Economies favoring double twist bunchers over tubular stranders are also evident in electrical drive power and the reduced level of spare parts and maintenance required. Double-twist bunchers, although generally capable of higher productive speed than the other types of stranding equipment, have not been used in the compressed stranded cable art because of numerous strand alignment problems, including loose strands, bird caging, wire crossovers, and inability to keep the core within the strand layer. In short, it has not been a practice to manufacture compressed stranded cable on double-twist bunchers.